In the processing of digital signals cases arise in which an output signal is required which has a phase position which is exactly coincident with the phase position of an input signal. Fundamentally, for this purpose a phase-lock loop may be used which attempts to keep the output phase always in coincidence with the input phase. An essential part of such a phase-lock loop is a phase detector which generates a control signal in dependence upon the deviation of the output phase from the input phase. Said control signal can then be employed to correct the phase deviation.
From "IEEE Transactions on Consumer Electronics", vol. CE-24, No. 1, February 1978, page 29 et seq., a phase detector is known which can be used in a phase-lock loop. This phase detector has however the disadvantage that it generates an undefined output signal when the phase of the input signal and the phase of the output signal differ only very slightly. This phase detector has a socalled "dead range" in which it does not operate in the manner it should and when used in the phase-lock loop produces disturbances which are referred to as "jitter".
The problem of the "dead range" in a phase detector of the aforementioned type is overcome in a phase-detector circuit which is described in "IEEE Journal of solid-state Circuits, vol. 24, no. 6, June 1989, pages 641 et seq. This known phase detector circuit is used in a control loop in which a certain fixedly predetermined phase offset is generated. This means that the phase-lock loop generates and keeps constant a constant phase difference between the input signal and the output signal corresponding to the phase offset so that the problem of the "dead range", present at very low phase deviations, is no longer encountered. However, such a phase detector cannot always be used because there are cases in which a fixed phase offset between the input signal and the output signal is not acceptable and identical phases must be present.